The invention relates to a rotatable pressure seal between a rotatable shaft and a body structure. The pressure seal is particularly but not exclusively suitable for use in a modulated bias unit used in drilling boreholes in subsurface formations. The invention will therefore be described in that context, but it will be appreciated that it is more widely applicable to many other situations where a rotatable pressure seal is required.
When drilling or coring holes in subsurface formations, it is often desirable to be able to vary and control the direction of drilling, for example to direct the borehole towards a desirable target or to control the direction horizontally within the payzone once the target has been reached. It may also be desirable to correct for deviations from the desired direction when drilling a straight hole, or to control the direction of the hole to avoid obstacles.
British Patent Specification No. 2259316 describes various arrangements in which there is associated with the rotary drill bit a modulated bias unit. The bias unit comprises a number of hydraulic actuators spaced apart around the periphery of the unit, each having a movable thrust member which is hydraulically displaceable outwardly for engagement with the formation of the borehole being drilled. Each actuator has an inlet passage for connection to a source of drilling fluid under pressure and an outlet passage for communication with the annulus. A selector control valve connects the inlet passages in succession to the source of fluid under pressure, as the bias unit rotates. The valve serves to modulate the fluid pressure supplied to each actuator in synchronism with rotation of the drill bit, and in selected phase relation thereto whereby, as the drill bit rotates, each movable thrust member is displaced outwardly at the same selected rotational position so as to bias the drill bit laterally and thus control the direction of drilling.
The selector control valve is located within a cavity in the body structure of the bias unit and is operated by a shaft which is rotatable relative to the body structure. Drilling fluid is supplied to the cavity through a choke and consequently there is a significant pressure difference between the interior of the cavity and a central passage where the main part of the shaft is located. In order to accommodate this pressure difference a rotatable pressure seal must be provided between the shaft and the body structure of the bias unit. The pressure seal must operate reliably under conditions of high pressure and temperature, and must be able to resist the highly abrasive effect of the drilling fluid. It must also operate under low torque. The present invention therefore provides a novel form of pressure seal which is particularly suitable for use in a modulated bias unit of the kind described, although it may also be suitabIe for use in other situations where a reliable rotatable pressure seal is required.